(from the shape of the Gr. letter A, delta, originally used of the mouth of the Nile), a tract of land enclosed by the diverging branches of a river's mouth and the seacoast, and traversed by other branches of the stream. This triangular tract is formed from the fine silt brought down in suspension by a muddy river and deposited when the river reaches the sea. When tidal currents are feeble, the delta frequently advances some distance seawards, forming a local prolongation of the coast.